Implantable electrodes are known in the art to suffer from problems of corrosion, specifically faradic effects, due to, inter alia, high energy transfer across the electrodes. When implantable electrodes are used as myocardial pacing electrodes, the faradic effects manifest themselves as electrode corrosion and dangerous build-up of metal deposits in the myocardial tissue contacting the electrode. Attempts have been made in the prior art to reduce the corrosion problem by using specific inert conductors as a substrate, and covering the substrate with different types of corrosion-resistant or corrosion-reducing coatings.
U.S. Pat. No. 5,654,030, to Munshi and Bonnerup, which is incorporated herein by reference, describes a method for making implantable electrodes for cardiac stimulation. The method comprises coating a cleaned valve metal surface with one or more metal oxides selected from the oxides of ruthenium, iridium, titanium, and tantalum. (Valve metals are titanium, tantalum, niobium, hafnium, zirconium, and tungsten.) The electrodes are intended for permanent implantation, i.e., implantation over a time period of the order of years, and can be removed only by an invasive procedure.
The inventors state that stimulation requires that an electric field of adequate field strength and current density be imposed on the excitable myocardial tissue in the vicinity of the electrode to initiate rhythmic contractions. The minimum electrical pulse necessary to produce such contractions is referred to as the stimulation threshold, and the inventors define an electrode efficiency by saying that the greater the efficiency of the electrode to generate contractions, the smaller is the amplitude and/or duration of the pulse required to exceed the threshold. The inventors further state that in all types of stimulation electrodes, the electrode itself must be both chemically corrosion resistant and mechanically stable enough to withstand chronic application, that it must possess a high "charge capacity," and that it must also inject a substantial level of electric charge into the tissue to be stimulated.
U.S. Pat. No. 5,385,579, to Helland, which is incorporated herein by reference, describes a myocardial electrode formed from titanium, platinum, or platinum iridium which is intended for permanent implantation. The electrode may be coated with a particle coating intended to enhance electrical efficiency and to help minimize fibrotic tissue growth response. Examples of particle coatings are platinum black, metal oxides, and metal nitrides.
Temporary/removable implantable electrodes, i.e., electrodes which are intended to be implanted in excitable tissue for a time period of the order of days, are also known in the art. These electrodes are commonly used after open heart surgery, for example, and may be removed from the heart tissue without further surgery by pulling them firmly through the chest wall.
PCT patent applications PCT/IL97/00012, PCT/IL97/00235 and PCT/IL97/00236, which are incorporated herein by reference, describe apparatus and methods of excitable tissue control (referred to herein as ETC) of tissue such as cardiac muscle. The term "excitable tissue control," in the context of the present patent application and in the claims, refers to application of electrical signals that do not induce activation potentials in cardiac muscle cells. Rather, such signals affect the response of the heart muscle to the action potentials, by modulating cell contractility within selected segments of the cardiac muscle. Such signals are also referred to as "non-excitatory" signals.
Typically, ETC signals are of significantly longer duration and have a significantly higher current than excitatory electrical stimulation pulses, and so transfer substantially more energy to cardiac muscle than excitatory electrical stimulation pulses.
Myocardial stimulation, both non-excitatory and excitatory, is typically performed using either a unipolar or a bipolar mode. In the unipolar mode, one cardiac electrode with its associated lead is used as a stimulation electrode, and the current return path does not comprise a second cardiac electrode. For example, a conducting body of an implanted pacemaker could be used for the current return. In the bipolar mode, two separate electrodes, with respective associated leads, are used to provide a stimulation and a return terminal. The unipolar mode has the advantage of only requiring one electrode, but suffers from the disadvantage that because the current path is not well-defined, the method can easily be affected by external changes in conditions, such as a patient moving. The bipolar mode has the advantage of a well-defined current path, at the expense of utilizing two electrodes.
In addition to being used for stimulation, a pacing electrode may also be used as a sensing electrode, in order to measure electric potentials at the site where the electrode has been positioned.